The purpose (theme) in the poem "Blackberry-Picking" written by the poet Seamus Heaney is embracing all that is bountiful, fresh, wonderful, and beautiful in life and enjoying it with exuberance. The poem is a metaphor on living life to the fullest and not wanting anything of beauty and wonder in life to fade away. The theme of the poem is "tasting the richness of life", just as one tastes the richness of a fully-ripened blackberry.
The poem indicates:
We hoarded the fresh berries in the byre.
This is akin to us taking hold of life and hording all the great experiences that we can.We seek to enjoy and be productive and creative in life - and then to have the memories - hoarded in our mind - of all these great and significant times.
However, the poet indicates that life does not always leave us with great memories or experiences. Sometimes, what we treasure is lost, or decays in some way, and is not as beautiful to us as it once first was. This is indicated in this line concerning the plump blackberries that were picked:
That all the lovely canfuls smelt of rot.
Each year I hoped they'd keep, knew they would not.